<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <button id="btn">单击</button>
    <script>
        var button = document.getElementById('btn')
        button.onclick = function () {
            alert('hello!')
        }
    </script>
</body>
</html>